The paper is devoted to research the contribution of ASE in the MOPA system output signal and time dependence of the signal-to-noise ratio. The results demonstrate how the output signal depends on time delay between ASE of power amplifier and laser radiation of master oscillator. The time delay (td) varied over the wide range (‒36; +24) ns, where «+» sign that the input signal enters the quantum amplifier (power amplifier) before the formation of its ASE. The light waveforms (ASE, input signal, output signal) and power dependencies are presented. The contribution of ASE in each case is determined. The results allow to estimate the signal-to-noise ratio. The maximal value of the output radiation power was 3,87 W at time delay td = + 4.7 ns, when the first maximum of input signal is approximately consistent with the ASE front.